1. Field of the Invention
The present invention relates to an acoustic sensor and an ultrasound probe.
2. Description of the Related Art
A conventional ultrasound diagnosis apparatus irradiates the interior of a subject with ultrasound waves, receives echoes reflected from the interior and analyzes the reflected echoes to examine the internal structure of the subject. Ultrasound diagnosis apparatuses are widely used for various purposes such as medical examination and inspection of the interior of buildings and structures since ultrasound diagnoses can nondestructively and noninvasively determine the states of subjects.
An ultrasound diagnosis apparatus converts received ultrasound waves into electric signals having strengths in relation to the intensities of the received ultrasound waves, and acquires the electric signals. An acoustic sensor for receiving such ultrasound waves includes transducers having piezoelectric elements. The sound pressure of ultrasound waves causes the piezoelectric elements to be mechanically deformed (expansion and contraction). The ultrasound diagnosis apparatus converts the deformation of each piezoelectric element into an electric signal (the amount of electric charge) having a strength in relation to the amount of the deformation, and detects the converted electric signal. In such a conventional acoustic sensor, transducers containing piezoelectric elements are formed into a plate or thick film or a multilayer substrate with a thickness of 10 μm or more in ordinary cases and 100 μm or more in general by, for example, a thick film coating technique. The ultrasound diagnosis apparatus detects deformation of each piezoelectric element in the direction of thickness, which is caused by ultrasound waves incident on a surface of this plate-like transducer, to determine the intensities of the ultrasound waves.
The piezoelectric element contains a ferroelectric material, such as lead (Pb) zirconate titanate (PZT). The ferroelectric material, which has polarization characteristics with hysteresis, has been used for nonvolatile memory (ferroelectric random access memory, FeRAM). U.S. Pat. No. 3,832,700 discloses a ferroelectric memory having a ferroelectric thin film between a gate electrode and a channel region separating a source and a drain. A voltage is applied to the ferroelectric thin film to alter electrical conductivity in the channel region so that the polarization characteristics corresponding to either of two values are kept. The state of conduction between the drain and the source is measured to read the two values.
Development of ultrasound diagnosis apparatuses with higher resolution and sensitivity needs ultrasound probes with higher precision. Unfortunately, a more complicated production process, a larger acoustic sensor, and increased process and labor costs are inevitable to acquire more efficiently electric signals from a piezoelectric component in an acoustic sensor used for receiving ultrasound waves in a conventional ultrasound probe in ordinal manners.